Process for start-up of fiber spinning solutions

ABSTRACT

A METHOD FOR THE START-UP OF A FIBER-FORMING ACRYLONITRILE POLYMER SOLUTION SPINNING OPERATION WHEREIN DURING SPINNING THE SPINNING SOLUTION IS MAINTAINED UNDER SUPERATMOSPHERIC PRESSURE AND AT A TEMPERATURE ABOVE THE ATMOSPHERIC BOILING POINT OF THE SOLVENT COMPRISING PREFILLING THE SPINNERET WITH A HIGH BOILING SOLVENT IN AN AMOUNT SUFFICIENT TO MAINTAIN THE SPINNERET FILTER SUBSTANTIALLY SATURATED WITH SOLVENT DURING THE START-UP UNTIL SPINNING SOLUTION FLOW COMMENCES.

United States Patent Office 3,707,588 Patented Dec. 26, 1972 3,707,588 PROCESS FOR START-UP F FIBER SPINNING SOLUTIONS Kenneth C. Campbell, Rock Hill, S.C., Donald E. Robinson and R. Russell Rhinehart, Charlotte, N.C., and Wade J. Truesdale, Rock Hill, S.C., assignors to Celanese Corporation, New York, NY. No Drawing. Filed Dec. 28, 1970, Ser. No. 102,196

Int. Cl. B28b 7/04 US. Cl. 264-39 7 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The present invention relates to a spinneret start-up procedure in the spinning of acrylonitrile polymeric shaped articles such as fibers and filaments wherein the solvent is above its atmospheric boiling point. More particularly, the invention also relates to the use of a high boiling solvent in a dry spinning system during start-up of a spinneret to prevent or hinder skin formation with the over-all objective to permit quicker and easier spinneret jet start-ups and increase the percentage of successful starts.

The spinning of acrylonitrile polymer solutions into shaped articles such as filaments is sometimes preferably effected utilizing solvents which form polymer solutions only at temperatures above the atmospheric boiling temperature of the solvent. In order to maintain solubility of the polymer in such solvents, the polymer solution is maintained at elevated temperatures and superatmospheric pressures. The start-up of spinneret orifices utilizing such systems is usually extremely difficult because, unless sufficient pressure is maintained on the polymer solution, the solvent rapidly vaporizes at the solution temperature, thereby precipitating the polymer and blocking the orifices and spinneret filter.

In the start-up of such systems, considerable feed lines, spinneret manifolds, and the spinnerets themselves must be fed with the polymer spinning solution while retaining it under sufficient pressure to inhibit solvent boiling and consequent solidification of the polymer. This pressure is provided by prefilling all parts of the spinning system which must be filled with dope during spinning, such as lines, pumps, etc., with a solvent at process temperature. The resulting vapor pressure of the solvent and the solvent itself prevent the formation of higher solids concentration dope (skins). When a pressurized dope system is opened to the spinnerets and extrusion does not begin almost immediately, the solvent rapidly evaporates from the polymer solution and escapes through the spinneret and filters much faster than the solution can be forced through the distribution system to the spinneret. The evaporation causes insoluble polymer skins to form on the forward mass of polymer solution, which subsequently blocks up solid against the filter and/or spinneret orifices. Once the associated system, including the spinneret, is filled with polymer solution and spinning begun, the back pressure from the dope extruding through the spinneret orifices is normally sufiicient to enable retention of the polymer solution in the spinning system under the required pressure, thereby retaining the desired fluidity.

SUMMARY OF THE INVENTION The present invention is particularly applicable to spinning solutions of acrylonitrile polymers comprising at least about weight percent of an acrylonitrile polymer and up to about 15 percent of an ethylenically unsaturated compound copolymerizable therewith, in acetonitrile or acetonitrile and up to about 40 weight percent water based on the weight of solvent. However, the invention is broadly applicable to any dope system wherein dope flashing and skin formation is a problem during spinneret start-up but is particularly useful in a pressurized system wherein the spinning solution solvent is maintained at a temperature above its atmospheric boiling point.

It is an object of the present invention to provide a process which permits the quick start-up of a pressurized acrylonitrile polymer spinning system without requiring the use of a solvent bleedoff or pressure relief valve to commence successful spinning operations. It is another object of the present invention to provide a method for quickly and easily starting-up an acrylonitrile polymer spinning system which precludes excessive pressure surges which can develop when a pressure relief valve is closed just prior to start-up. These and other objects will become apparent to those skilled in the art from a description of the invention which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the invention, a method is provided for the start-up of a pressurized fiber-spinning system for a solution of an acrylonitrile polymer, preferably in a solvent comprising acetonitrile or acetonitrile and water, wherein said solution is maintained at a temperature above the atmospheric boiling point of said solvent. The fibers are spun through an orifice or orifices from said solution while maintaining the solution under superatmospheric pressure, the release of which pressure can cause the rapid evaporation of the solvent and excessive concentration or solidification of at least a portion of the acrylonitrile polymer. The present invention comprises substantially saturating the spinneret filter with another solvent, i.e., a high boiling solvent having an atmospheric boiling point above the spinning temperature, preferably about to 225 degrees centigrade, such as ethylene carbonate, preferably maintained at a temperature of from about 80 C. to 160 C., to preheat the spinneret to about the spinning temperature, i.e., a temperature of from about 80 C. to 160 0, preferably 110 C. to C. and dissolve the skins formed, which cause filter and spinneret orifice blockage. The spinneret is placed onto the spinning column while maintaining the system spinning temperature and the spinning solution is then passed through the system to the spinneret to replace the solvent in the spinneret and subsequently spinning commences.

Because the invention is particularly applicable to acrylonitrile polymers in low-boiling solvents, it will be described hereinafter more particularly with reference to such polymers. As referred to herein, acrylonitrile polymers are preferably those containing at least 85 percent acrylonitrile by Weight. These polymers can be acrylonitrile homopolymers as well as copolymers, ter-polymers, multipolymers and the like wherein up to about 15 percent of the polymer is another ethylenically unsaturated compound copolymerizable with the acrylonitrile. Such materials can be monomers or polymers which are copolymerizable with acrylonitrile and added to modify and/ or enhance certain characteristics of the acrylic polymer. Often, the material copolymerizable with the acrylonitrile contains a chemical group which increases the basic dyeability of the resulting polymer. Such dye enhancing compounds normally contain a sulfur or phosphorous group in the ethylenically unsaturated chemical entity copolymerizable with the acrylonitrile. Typically, such sulfur or phosphorous containing compounds are added in an amount of about 0.1 up to about percent by weight of the total polymer composition while the other modifying substance, if any, is added in an amount of up to about 14.9 percent. Typical ethylenically unsaturated monomers copolymerizable with acrylonitrile are methyl acrylate, vinyl acetate, vinylidene chloride, methyl methacrylate, methallyl alcohol, vinylidene cyanide, styrene sulfonic acid materials, sodium methallyl sulfonate, mixtures and partial polymers thereof and the like as are well known to those skilled in the art. The polymers are polymerized by conventional methods such as solution or suspension polymerizations, as well known in the art.

The degree to which the polymer is polymerized is dependent on the end use for which the polymer is intended. Thus, for spinning acrylic fibers, the polymer is preferably polymerized to an intrinsic viscosity (I.V.) of about 0.9 to 2.0 or more, and more preferably about 1.2 to 1.8 (in N,N-dimethylformamide at 25 degrees centigrade). Of course, higher I.V.s can be used but they result in higher viscosities for given solvent concentrations. For films, molded products, extruded non-fiber products and the like, different I.V.s may be more desirable.

In the solution spinning of fibers, it is preferable to use a high solids concentration in the spinning solution. With the preferred solvent system, such high solid concentrations are readily obtained using the acrylic polymers having I.V.s in the normal range used for acrylic fiber spinning, that is, about 1.2 to 1.8. In particular, solutions of a solids content of about 30 to 70 percent by weight are readily obtained. The more preferred spinning solutions are obtained in the solid range of about 30 to 50 percent with polymers having I.V.s of about 1.2 to 1.8 or more. With lower I.V. polymers, such as about 0.9 to 1.2, higher solids contents up to about 70 percent or more can be used.

The spinning solutions are formed by mixing the desired solid amount of acrylonitrile polymer with acetonitrile by itself or with water in an amount up to about 40 percent of the total solvent portion. While a water addition need not be used, it has been found that the addition of water, particularly in the range of about 18 to 27 percent by weight of the acetonitrile solvent portion, lowers the gelation point of the resulting solution, thereby enabling greater flexibility and control of spinning, molding or extruding temperatures.

The acrylic polymer and acetonitrile solvent portion are heated under superatmospheric pressure to a temperature above the atmospheric boiling point of the solvent and below the polymer degradation temperature (i.e., 80 to 160 degrees centigrade) wherein the polymer passes from a solid into a slurry state and subsequently into a liquid state. Having reached the liquid state, the temperature can then be lowered to about 80 to- 85 degrees centigrade without gelation, if pressure is maintained. The gel temperature on cooling is normally above or near the boiling point of the acetonitrile under process pressure depending on the amount of water present, the polymer the acrylonitrile content, the solids content and the The superatmospheric pressure utilized to form the solution is preferably at least equal to that required to maintain the acetonitrile solvent portion in substantially the liquid phase. For example, at 130 degrees centigrade and 34 percent solids, the vapor pressure of the dope is approximately 58 p.s.i.g. At 90 degrees centigrade, the vapor pressure is only 20 p.s.i.g., however, higher pressures such as 100 to 300 p.s.i.g. can conveniently be used.

In furtherance of the described method, a preheated spinneret with its filter substantially saturated until spinning solution flow begins, with a high boiling solvent (e.g., ethylene carbonate or N,N-dimethylformamide) is provided, the spinneret comprising a spinneret plate having at least one orifice therein and polymer feed means communicating with the plate and orifices. After the polymer feed means is turned off, the preheated spinneret, filled with at least enough solvent to maintain the spinneret filter substantially saturated with solvent until spinning commences, is attached. Alternatively, the spinneret does not have to be preheated, but can be allowed to heat in the column, e.g., via convection and conduction from the hot air, until the desired spinneret temperature is achieved. The spinneret may intermittently spray solvent from several spinneret orifices initially until the dope fills the spinneret cup and dope pressure increases, but extrusion usually begins within about 2 minutes. Stable spinning commences after high viscosity portions of the dope (skins) resulting from dope flashing, are extruded or redissolved.

The present invention is particularly useful for the start-up of spinning units ranging from single pilot plant units up to commercial spinning systems comprising several hundred or more spinnerets. The method also permits charging of the spinnerets without cooling down the spinning dope.

In the start-up of a spinning operation utilizing temperatures above the boiling point of the solvent used to solution the acrylonitrile polymer to be spun, the spinning system (all filters, piping, etc.) is first filled or partially filled with the acetonitrile/ water solvent used in the process and then heated to process temperature to prevent entering dope from flashing. The solvent has a sufficiently high vapor pressure under the spinning or extruding conditions such that it can rapidly flash from the dope or polymer solution, thereby rigidifying or gelling the polymer. Thus, the polymer can rigidify both because of the flashing of solvent and because of the lack of solubility at a reduced temperature.

For instance, when acetonitrile/water in a ratio of about 70 to weight percent acetonitrile and about 20 to 30 weight percent water is used as a solvent for a high acrylic polymer (containing more than percent acrylonitrile), the solvent is heated to a spinning temperature of from about 80 to 160 degrees centigrade, preferably from about 110 to 150 degrees centigrade. This creates an autogenous pressure of about 40 to 50 pounds per square inch gauge, which pressure is required to keep the solvent in the liquid state.

Polymer solution, also at an elevated temperature and pressure sufiicient to maintain the polymer solution in a fluid condition, is then pumped through the spinning syster to completely replace the solvent which was first added to the system. A spinneret assembly is prefilled with a high boiling acrylonitrile solvent, such as ethylene carbonate, and preferably preheated to a spinneret temperature substantially equal to the spinning temperature, i.e., from about to about 160 degrees centigrade, preferably from about to degrees centigrade, and is attached to the spinning system.

The invention will be more fully described by reference to the following example which illustrates certain preferred embodiments of the present invention. Unless otherwise indicated, all temperatures are in degrees centigrade and all parts are by weight.

'EXAMPLE In accordance with the invention, an acrylonitrile copolymer comprising about 94.5 percent acrylonitrile, about 5 percent methyl/acrylate and about 0.5 percent sodium methallyl sulfonate, polymerized to an intrinsic viscosity of about 1.4 and a molecular weight of about 110,000 was solvated with acetonitrile by placing 36 pounds of the acrylonitrile polymer into a pressure vessel together with 51.2 pounds of acetonitrile and 12.8 pounds of water. The resulting mixture comprised 36 percent acrylonitrile copolymer solids, 12.8 percent water and 51.2 percent acetonitrile, by weight, wherein the acetonitrile/water were in a proportion of about 80/20 acetonitrile/water, by weight.

The pressure vessel was sealed and subsequently slowly mixed for 8 hours at room temperature, then the temperature was raised slowly to 110 C. at which point the material in the vessel changed from a slurry through a gel-like phase into a homogeneous fiuid solution (dope). The mixture was then mixed for 18 hours more at 110 degrees centigrade while maintaining a pressure of about 50 p.s.i.g., creating a dope viscosity of about 100 poise. While maintaining the temperature and pressure, the dope was recycled through a 100-micron filter.

The filtered dope was passed through a 20 micron filter, while maintaining the temperature and pressure, then through a spinning dope pump to a pressure relief valve. The following procedure was utilized for starting up the spinneret to minimize down-time of the spinning dope pumps and possible resulting pump failures:

(1) The spinning pump and the valve leading from the spinning pump to the pressure relief valve were simultaneously closed.

(2) The pressure relief valve was removed.

(3) The residual dope was cleaned from the column fitting which was subsequently rinsed with N,N-dimethyl formamide.

(4) A new spinneret, which has been filled with sufiicient ethylene carbonate to substantially saturate the spinneret filter and to maintain substantial saturation until spinning solution flow commences, is placed (screwed) onto an adapter on the spinning column. A small amount of ethylene carbonate drips out of the orifices in the spinneret face, as the spinneret assembly is allowed to heat up to about 110 degrees centigrade in the spinning column-the time required being about 15 minutes. It is believed that an air pocket forms between the spinneret face and the filter, which air pocket is saturated with ethylene carbonate vapors.

(5) The valve leading to the spinneret is opened and the spinning pump simultaneously or immediately thereafter stated. Solvent intermittently sprays out from the spinneret orifices for several seconds and then spraying stops. Most solvent which was above the level of the filter or actually in the spinneret cup is pushed out by the forward moving and flashing dope.

(6) Bubbly appearing dope begins to fill the filter and the spinneret cup. Bubbles and high viscosity dope regions (skins) are initially present and the spinneret dope filter initially bulges. The dope fills the spinneret cup but does not immediately begin extrusion. As the pressure builds up to about 800 p.s.i.g., dope begins to extrude from the spinneret orifices, with a few orifices intermittently extruding dope for a short time. Bubbles (solvent vapor pockets) in the spinneret cup are redissolved because of increased pressure and high viscosity zones or skins which remain disappear within a few minutes by either redissolving or by being extruded.

Alternatively, the spinneret and the spinneret filter may be preheated and substantially saturated with ethylene carbonate prior to being attached to the spinning column. Using this method, the dope flow can be stopped, the pressure relief valve removed, the column cleaned, a new preheated jet attached, and extrusion started within 2 minutes and usually within 30 seconds.

The solvent that is utilized to prefill the spinneret assembly preferably should be a high boiling acrylonitrile solvent to minimize evaporation. This solvent performs at least two functions: (1) dissolves or breaks up skins to facilitate their passing through the spinneret filter and orifices and (2) may help supply a back pressure on the flashing dope by providing a liquid resistance through the spinneret orifices, as opposed to the vaporized solvent blowing through the spinneret.

In addition to ethylene carbonate (which boils at 126 C.), examples of other solvents which are used with substantially equal results are N,N-dimethylformamide (boils at 153 C.), dimethylsulfoxide (boils at 189 C.), and butyrolactone (boils at 206 C.).

While there have been described various embodiments of the present invention, the methods and apparatus described herein are not intended to be understood as limiting the scope of the invention as it is realized that changes therein are possible. It is intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner. It is intended to cover the invention broadly in whatever form its principles may be utilized being limited only by the appended claims.

What is claimed is:

1. A method for the start-up of a pressurized fiber spinning system for a solution of a fiber-forming acrylonitrile polymer, in a solvent wherein during spinning the spinning solution is maintained at a temperature above the atmospheric boiling point of the solvent and said fibers are spun through at least one orifice from said solution while maintaining said solution under superatmospheric pressure, the inadvertent release of which pressure would cause the rapid evaporation of said solvent and the solidification of said polymer, comprising prefilling the spinneret with another acrylonitrile polymer solvent in an amount sufiicicnt to maintain the spinneret filter in a substantially saturated condition until spinning solution flow commences, said solvent having an atmospheric boiling point above the spinning temperature, maintaining the spinning system at the spinning temperature and placing the prefilled spinneret onto the spinning column, passing spinning solution through the spinning system to said spinneret to replace the solvent in said spinneret which spinneret has been heated to about the spinning temperature and subsequently commencing spinning of a fiber.

2. The method of claim 1 wherein the spinneret is preheated to about the spinning temperature prior to being placed on the spinning column.

3. The method of claim 1 wherein the spinneret is heated to about the spinning temperature on the spinning column subsequent to being placed in said column.

4. The method of claim 1 wherein the solvent used to prefill the spinneret has an atmospheric boiling point of from about 125 to 225 degrees centigrade.

5. The method of claim 4 wherein the solvent consists essentially of ethylene carbonate.

6. The method of claim 1 wherein the spinning temperature ranges from about to 160 degrees centigrade.

7. The method of claim 1 wherein the spinning temperature ranges from about to degrees centigrade.

References Cited UNITED STATES PATENTS 1,671,878 5/1928 Topham 18-8 D 1,942,632 1/1934 LoWen 18-8 SS 2,330,932 10/1943 Taylor et al. 264-469 2,872,702 2/1959 Dunlap et al 264-169 3,428,724 2/ 1969 Billheimer 264-207 3,461,193 8/1969 Gilardi 26453 3,436,789 4/1969 Hays 264--39 2,706,674 4/1955 Rothrock 264-206 JAY H. WOO, Primary Examiner US. Cl. X.R. 

